CVD diamond radiation detector

ABSTRACT

The process is an arc jet CVD diamond deposition process with very low methane, less than 0.07%, and the addition of water. The resulting material has is characterized by a narrow Raman peak, a relatively large lattice constant, and a charge carrier collection distance of at least 25 microns.

The present invention was made with Government support, and thegovernment has certain rights in the invention.

This is a divisional of application Ser. No. 08/094,826 filed on Jul.20, 1993, now abandoned.

TECHNICAL FIELD

The invention relates generally to electronic radiation detectors andmore particularly to detectors which make use of chemical vapordeposited diamond film.

BACKGROUND OF THE INVENTION

One type of solid state radiation detector has a strong electric fieldestablished between two electrodes within free-standing insulatingmaterial. When the insulating material is exposed to radiation ofsufficient energy to bring electrons or electron-hole pair carriers intothe conduction band, the carriers are swept to the electrodes by theelectric field. Their arrival at the electrodes can be measured by anelectronic signal detection device connected to the electrodes.

It has already been recognized that diamond in general is a particularlyadvantageous material for use in a solid state particle detector,especially for SSC (superconducting supercollider) particle physicsresearch, because diamond is much more resistant to radiation damagethan are alternative detector materials, such as silicon with a P-Njunction. SSC accelerators produce an intense amount of radiation attheir collision points. Silicon detectors suffer crystal structuredefect damage in such an environment which leads to an increased leakagecurrent and a decreased pulse height in their output signal.Furthermore, in silicon, the maximum field that can be applied beforeavalanche breakdown is about 10³ V/cm. This limits the charge velocityto approximately 10⁶ cm/s, so that the collection time is at least 20 ns(nanoseconds) for a detector with a thickness of a few hundred microns.However, such a long collection time can lead to difficulty ininterpreting results from an SSC accelerator, since in such anaccelerator the beam collisions occur on a timescale of less than 20 ns.

It has also already been recognized that CVD (chemically vapordeposited) diamond film is a particularly advantageous material for thedetection of particle radiation. Diamond film of the CVD type can bemade with lower impurity levels than natural diamond or diamond made bya high-temperature high-pressure process and can be readily provided inthe wafer geometry preferred for particle detectors.

For making a CVD diamond detector, a free-standing CVD diamond film,typically several hundred microns thick, is metallized with acomplementary electrode pattern on each of its faces. The dimensions ofthe electrode pattern will determine the spatial resolution of thedetector. A voltage is applied between the electrodes, so that theelectrons and holes will be accelerated to their respective, oppositepolarity electrodes to produce a signal. In order to achieve anacceptable signal-to-noise ratio, it is necessary to avoid having theelectrons and holes trapped by defects in the material. The collectiondistance "d" is the average distance that electrons and holes driftunder the applied electric field before recombination at a trappingsite. The collection distance d has also been found to be equal to theproduct of the carrier mobility, the carrier lifetime and the appliedelectric field. Early CVD diamond films had a collection distance ofless than one micron, with both the mobility and lifetime being muchlower than for natural IIa diamond. For a calorimeter-type particledetector, a minimum performance level is a collection distance of 25microns, although 50 microns is considered most desirable. The highestvalue achieved thus far has been 15 microns with a mobility of 4000 cm²V⁻¹ s⁻¹ and a lifetime of 150 ps (picoseconds), both at an applied fieldof 200 Volts per centimeter. The lifetime may be limited by defects suchas dislocations, stacking faults, impurities and twins. There is,therefore, a need for a diamond material which will permit theachievement of a greater collection distance d for particle detectors.

SUMMARY OF THE INVENTION

In accordance with the present invention, a novel CVD diamond filmmaterial which is made by a novel process exhibits greatly improvedcollection distance when used as a particle detector. The material ismade by an arc jet process which includes a very low carbon source gasconcentration, together with the addition of an oxidant source, such aswater, to the process gases.

The CVD diamond material of the present invention exhibits asubstantially improved collection distance for electrical carriersgenerated in it and is therefore an improved material for electronicpurposes in general.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, sectioned, front view of a typical arc jetdeposition apparatus known in the art which has been modified by theaddition of water injection means for practicing the present method.

FIG. 2 is a schematic, cross-sectional view of a solid state particledetector device made with the diamond material of the present invention.

DETAILED DESCRIPTION PROCESS

For description of a preferred embodiment of the process in accordancewith the present invention, reference is made to the schematicrepresentation of FIG. 1, which shows an arc jet apparatus 10. Theapparatus 10 includes a cathode member 12 at the top end of a hollowbarrel 14 in a metal jacket member 18 having an annular space 19suitable for holding a fluid coolant. The barrel 14 and jacket member 18are surrounded by a fluid-cooled magnetic coil assembly 20.Longitudinally spaced at the end of the barrel 14 opposite that of thecathode 12 is an anode member 22 having a central opening aligned withthe axis of the barrel 14 and leading through a nozzle 24 into anevacuated deposition chamber 26 which has a cooled deposition substrate28 spaced from the end of the nozzle 24. A gas injection means 30 islocated to inject gas into the barrel 14. Other gas injection means 32are located in the vicinity of the anode 22.

In the operation of the arc jet apparatus 10, hydrogen gas is injectedthrough the injector tubes 30 and 32 at a predetermined rate. Morehydrogen gas, mixed with methane, is injected through the tube 32. Theconcentration of methane is based on the total percentage of methaneinjected as a volume percent of the total gas injected through bothtubes 30,32. A direct current arc is struck between the cathode 12 andanode 22. The enthalpy of the gas in the barrel is adjusted by controlof the arc power to result in the desired temperature of the substrate28, which is heated by the gas impinging from the nozzle 24. At thisenthalpy, the hydrogen becomes partially dissociated hydrogen atoms. Themagnetic coil assembly 20 around the barrel 14 generates a solenoidalmagnetic field which has the effect of swirling the arc about the anode22 to reduce anode erosion.

The activated gas mixture traveling through the nozzle 24 enters theevacuated deposition chamber 26 and impinges on a fluid-cooleddeposition substrate 28 therein to form a diamond film on it. As themethane enters the activated gas through the tubes 32, it too becomespartially dissociated into unstable hydrocarbon radical species. A setof three aluminum oxide ceramic tubules 34 positioned in radial symmetrywith their ends in the deposition zone between the nozzle 24 and thesubstrate 28 are fed with water by a peristaltic pump, not shown. At thesubstrate 28, the hydrogen acts as a facilitating gas for the depositionof the carbon atoms from the activated hydrocarbon radicals as diamondcrystallites bonded to each other. The diamond crystallites consist ofcarbon atoms bonded chemically to each other by what is generallyreferred to as "sp3" bonds.

Apparatus of the arc jet type, such as the apparatus 10 described above,is known in the art, except for the water injection apparatus includingthe tubules 34. There are, of course variations is such apparatus and inthe methods of operating it. Therefore, many other parameters areinvolved in the deposition process. However, it is submitted that themost important ones are generally the enthalpy (kilojoules/gram), vacuumlevel (torr), substrate temperature (degrees Celsius), methaneconcentration (percent), and water injection rate. Given these parametervalues, the others can be determined for a given apparatus design andmethod of operation by the skilled operators familiar therewith withoutthe necessity of undue experimentation. Such parameters do not lendthemselves well to generalization, since they are dependent on specificapparatus design features.

The gases used must be highly pure with respect to certain elements.There should be an impurity level of less than 1,000 ppm (parts permillion) for substances other than hydrogen, carbon, oxygen, argon, andhelium. If the objective is to grow a free-standing diamond film, thedeposition substrate is preferably molybdenum which has been coated witha thin layer about 3 microns (micro-meters) thick of titanium nitride,such as by vapor deposition, to reduce the adherence of the diamond tothe substrate for better release of the film.

Diamond film samples were made on an apparatus essentially similar tothe jet apparatus 10 described above. In each case, the arc power wasbetween 20 and 40 kilowatts and the deposition rate was between 3 and 6microns per hour. The temperature of the substrate is in degrees C(Celsius).

    ______________________________________                                                     Sample                                                                        A    B      C      D    E    F                                   ______________________________________                                        deposition conditions                                                         chamber press. (torr)                                                                        12     12     12   12   12   12                                substrate temp. (°C.)                                                                 825    844    825  933  850  840                               % methane      .050   .052   .076 .050 .072 .050                              enthalpy (kJ/g)                                                                              32.9   31.8   35.4 34.5 50.4 35.3                              power in kW    29.6   28.6   31.8 31.2 31.6 31.8                              water in g/min.                                                                              0      0      0    0    2    2                                 O/C molar ratio                                                                              0      0      0    0    7    7                                 thickness (microns)                                                                          308    400    383  357  410  300                               analysis of deposited samples                                                 Raman FWHM (/cm)                                                                             --     2.8    4.6  6.5  --   2.9                               thermal cond. (W/mK)                                                                         1130   --     1230 1110 1430 1430                              collection distance                                                                          3      4      3    2    45   41                                (microns)                                                                     lattice constant                                                                             --     3.568  3.567                                                                              3.566                                                                              3.570                                                                              3.569                             (Angstroms)                                                                   ______________________________________                                    

The substrate temperature is in degrees Celsius as measured by apyrometer. The percent methane is the proportion by volume of themethane in the gas added through the tubes 30,32. The enthalpy is inkilojoules per gram. The power is the arc power in kilowatts. The waterinjection rate is in grams per minute. The O/C molar ratio is the molarratio of oxygen to carbon in the deposition zone between the nozzle 24and the substrate 28. The thickness is that of the diamond beingdeposited on the substrate 28. The Raman FWHM is in units of reciprocalcentimeters and is the full width at half the maximum of the Ramanscattering 1332/cm peak which is characteristic of diamond. The thermalconductivity was measured by the converging wave method. Such a methodis described, for example, in "Measurement of thermal diffusivity ofPolycrystalline Diamond Film by the Converging Thermal Wave Technique,"by G. Lu and W. T. Swann in Appl. Phys. Letters 59 (13), Sep. 23, 1991.It is generally recognized that there can be substantial variations inthermal conductivity measurements from method to method. The collectiondistance was measured by a particle-induced conductivity technique ofthe type described in "Particle-And Photo-Induced Conductivity In TypeIIA Diamonds" by L. S. Pan et al, Journal of Applied Physics, Jul. 15,1993. The samples were not subjected to a radiation annealing process ofthe type sometimes referred to as "pumping" or "priming," which wouldsignificantly increase the collection distance. It is a drawback of theannealing process, however, that it tends to result in drifting of thebaseline and is therefore troublesome in practical use. It is believedthat the local collection distance of a given quality material isdirectly proportional to the distance from the surface of the diamondwhich was in contact with the substrate during deposition. We havetherefore normalized all collection distances to a thickness of 400microns. The lattice constants were measured by standard x-raydiffraction means. Polishing of the surface of the diamond which was incontact with the substrate during deposition can also produce anincrease in the collection distance, but is a costly and difficultprocess because of the fragility and hardness of such thin diamond. Itis an advantage of the diamond material in accordance with the presentinvention that it has a collection distance long enough to permit itsuse in a particle detector device without annealing or polishing.

MATERIAL

The results shown in the above table permit some observations withregard to characteristics of diamond material with a long collectiondistance. It is noted, for example, that Raman line width appears to benarrower for materials with increased collection distance. Also, thereappears to be a correlation between a larger lattice constant and thecollection distance, with a lattice constant of 3.569 or greaterrepresenting a dramatic increase in the collection distance. The thermalconductivity also appears to be improved for the samples E and F withthe long collection distance.

The collection distances were measured with an electric field strengthof 10 kilovolts per centimeter. In order for the diamond material tohave a long carrier collection distance, it is essential that it besubstantially free from most crystal lattice defects. Since the defectsare microscopic, it is useful to assess their concentration by measuringcertain characteristics of diamond which have been found to provide someindication of the degree to which defects are present. Thesecharacteristics are Raman line width and the thermal conductivity.

The results show that the specimens made with the added oxidant exhibita much longer collection distance. Experience would also lead to aconclusion that samples E and F made with injected water are likely tocontain less than 100 ppm (parts per million) of conductivity-enhancingimpurities.

The Raman linewidth is the full line-width at half the maximum of the1332/cm frequency Raman scattering spectrum line of diamond. This widthgives an indication of the degree of ordering of the diamond. Theanalysis of the samples A-F show that diamond with larger Raman linewidths has much reduced collection distances. The examples show that anarrow Raman line profile, while perhaps not alone a sufficientcondition for determining that a material will exhibit a long collectiondistance, does appear to be associated with material having a longcollection distance.

We have also noted that only samples with relatively high levels ofthermal conductivity exhibit long collection distances, although highthermal conductivity does not by itself guarantee long collectiondistance.

While it has been previously suggested by others in the art that theaddition of oxygen, such in the form of water, to a combustion,thermionic, or microwave CVD diamond manufacturing process would have afavorable effect on the quality of the resulting diamond material, thediscovery of the present invention that the addition of oxygen, such aswith water, to an arc jet process with very low methane would result ina material with a substantially improved charge carrier lifetime was notknown before.

The diamond material in accordance with the present invention typicallyhas a collection distance of 35-50 microns. The mobility is 3000-4000cm² V⁻¹ s⁻¹ and the lifetime is over 1 ns. This increased lifetime ismuch higher than that previously reported as best in the literature forCVD diamond (150 ps) and is even higher than for natural IIa diamond(300-550 ps). The addition of water is seen to greatly improve thelifetime. Under identical conditions except for water, the diamond madewithout water had a lifetime of 120 ps while the diamond made with waterhad a lifetime of over 1 ns. Both had mobilities of 3000-4000 cm² V⁻¹s⁻¹.

The collection distance is measured by applying a voltage to theelectrodes on each side of the diamond and analyzing the signal after ithas been amplified by a charge-sensitive preamplifier and by a signalshaping amplifier. The initial particles to be detected can be from aradioactive source (e.g. strontium 90) or from a particle acceleratorbeam line. The collection distance is determined from ##EQU1## where"Q_(gen) " is the amount of charge generated by the ionizing radiation."Q_(meas) " is the measured charge and "t" is the diamond thickness."Q_(gen) " is calculated by normalizing the diamond pulse height to thesilicon pulse height (with corrections) or using a Monte Carlosimulation.

The mobility and lifetime are measured by UV transientphotoconductivity. One mm wide electrodes are deposited on the same sideof the diamond with a 1 mm gap between the electrodes. The gap isilluminated with 3 to 5 ps (picosecond) pulses from a 202 nmfrequency-multiplied Nd-YAG (neodymium-yttrium aluminum garnet) laser.These pulses are typically up to 20 μJ/pulse at 10 Hz. The UV pulsecreates electron-hole pairs and the subsequent current pulse is relatedto the carrier lifetime while the amplitude and total charge are relatedto the product of mobility and lifetime. In this test, only the top 2microns at the surface is sampled due to the intrinsic absorption of UVlight by diamond. In the particle-induced conductivity tests, theperformance of the entire diamond thickness is sampled. Since thematerial on the substrate side is poorer and has small grain size, theparticle-induced conductivity test gives a collection distance which issmaller than that deduced from the photoconductivity tests. Thedifference is typically a factor of two.

DEVICE

FIG. 2 of the drawings shows a particle detector 36 which features awafer 38 of CVD diamond according to the present invention provided withtwo ohmic contact metal electrodes 40,42 on its faces. The electrodes40,42 are connected to signal processing circuitry which includes avoltage source in series with a load resistor 46. An amplifier 48 isconnected to the electrode 40 via an isolation capacitor 50. Electrodescould alternatively be in the form of interleaved comb-like structureswhich are both on the same face of the wafer. Such device structures arepresently known in the art for use with diamond other than that of thepresent invention. The operation of the device is as described earlierin the discussion of such detector devices.

GENERAL CONSIDERATIONS

There is reason to assume that other oxidants, such as carbon dioxide,can be used in place of water for providing the oxidant used in theprocess. In addition, acetone, acetylene, and alcohols have beenreported as substitutes for methane as the carbon source gas which wouldalso contribute oxygen to the mix. These are expected to give similarresults to methane with water if the concentrations are adjustedappropriately: generally each atom of oxygen bonds firmly to one carbonatom, so that a molecule such as acetone (CH3COCH3) contributes about asmuch free carbonaceous species as two molecules of methane (CH4). Theterm "oxidant" herein is used to denote substances traditionallyconsidered in this class in the chemical arts. Tightly bonded moleculessuch as acetylene are less effective in producing diamond than aremolecules like methane. However, if the residence time of the acetylenemolecule is long enough, it may convert partially to more active speciessuch as methane in flight. If the acetylene (or any other molecule) isinjected into the arc, then it is substantially broken up and shouldcount as if the carbon were present as methane (unless oxygen ispresent). Thus, it would be expected that one could obtain resultssimilar to those described above in accordance with the invention by theuse of carbon source gas other than methane which includes one or moreoxygen atoms and is present in a concentration equivalent to that of themethane concentration disclosed herein in terms of the resulting activespecies. Sulfur and the halogens fluorine and chlorine could also beexpected to improve the collection distance by oxidizing impurities andattacking structural defects much as oxygen appears to do in thedeposition process. Therefore, the invention is not intended to belimited to the use of methane alone as the carbon source gas or wateralone as the oxidant source. However, water is a particularlyadvantageous oxidant source from the standpoint of convenience, cost,and safety considerations.

Similarly, while here the facilitating gas is hydrogen, it has beenshown by those skilled in the art that there may be other gases used tofacilitate the growth of diamond films.

What is claimed is:
 1. A polycrystalline CVD diamond particle detectordevice, comprising:a wafer of chemically vapor deposited diamondexhibiting a collection distance of at least 25 microns and electrodemeans in electrical contact with the wafer and adapted to generate anelectric field in the wafer when provided with a biasing voltage.
 2. Thedevice according to claim 1, wherein the wafer is of diamond having aRaman full width at the half maximum of the 1332 per centimeter Ramanscattering peak of less than about 5 per centimeter.
 3. The deviceaccording to claim 2 in which the diamond exhibits a collection distanceof at least 35 microns.
 4. The device according to claim 1 wherein thediamond wafer is unannealed.
 5. The device according to claim 1 whereinthe diamond wafer is unpolished.
 6. The device according to claim 5wherein the diamond wafer is unannealed.